In the process of manufacturing fibrous materials, particularly, cellulose-based materials, such as wood, paper, textile and other materials, moisture must be removed to a desired moisture content level, while maintaining a uniform moisture profile. Failure to do so can result in inferior and defective product. For example, in the process of drying green wood, typically using a kiln, free water from cell lumina will naturally be depleted first, while the bound water (bound to the wood via hydrogen bonds) saturating the cell walls will remain until all of the free water is removed. The moisture content remaining in the cell walls after the free water has been removed is referred to as the Fiber Saturation Point (FSP), and is typically between around 24 to 32% and could reach levels of approximately 70%. The FSP further defines the moisture content below which, as the wood is further dried, properties such as volume and strength are affected. As is the case in typical kiln drying, the outer surfaces will dry and consequently shrink faster than the interior portions of the wood. As a consequence of this relative shrinkage, the wood can crack and split (a defect generally referred to as “checking”). In addition, if the faster drying portions become too dry at any point during the process, the strength of the material can be altered and warping of the wood can occur.
To mitigate these problems, conventional kiln drying processes include alternately heating and drying the wood with a moisture-removal mechanism, such as circulating unsaturated air to remove the moisture as it evaporates off, and rewetting the wood to redistribute the moisture in order to restore a more uniform moisture profile throughout the bulk of the material. For the heating process, various conduction, convection, and radiation heating methods have been used, including electrical heating means, steam-heated heat exchangers, and solar energy. In this so-called charging phase of a conventional kiln, as the temperature rises in the kiln, the wood surface is typically “overdried” so that the moisture content of faster drying portions is less than that of the desired final product. During the discharge or rewetting phase, the relative humidity in the kiln rises as the temperature falls. This slows the surface drying rate and equalizes the moisture profile through the wood. Air is also constantly circulating through the kiln and around the wood to remove moisture and assist in drying the wood. The rewetting and drying are typically further controlled by regulating the temperature and humidity of the air circulating in the kiln.
There are many disadvantages using such conventional kilns including possible loss of the strength of the wood due to overdrying of the outer surfaces, the possibility of other defects in the wood due to the difficulty in maintaining a uniform moisture profile, high energy consumption, and the release of pollutants into the atmosphere. In addition, the long drying times and relatively small amount of wood that can be processed in each batch cause a bottleneck in the entire production process.
Other known traditional methods of drying hardwood timbers can take several months requiring controlled conditions to prevent damage to the timbers. Such known drying processes are controlled so that the loss of moisture is gradual and the timber or wood shrinks evenly. These processes can take as long as 60 days.
To overcome some of the disadvantages of conventional methods of drying hardwood and other methods of kiln drying, early attempts were made to use microwave radiation to try to remove moisture from wood. However, such early attempts failed due to collection of moisture in the microwave emitter, causing it to malfunction, and on the surface of the material, preventing further removal of moisture from within the bulk of the material.
A method of using microwave to pretreat wood prior to applying conventional kiln drying techniques is disclosed in U.S. Pat. No. 7,089,685 to Torgovnikov, et al. (referred to hereinafter as the “'685” patent). The '685 patent discloses subjecting a surface of wood to microwave at 0.1 to 24 GHz to provide a modified wood zone on the exterior having increased permeability relative to the untreated core volume of the wood. The '685 patent discloses that this microwave pretreatment reduces the time required for the subsequent drying process using a conventional kiln. A variation of the kiln drying process uses RF in vacuum (“RF/V”) to heat a stack of wood volumetrically, causing a more uniform moisture profile in the heating process, and causing the kiln environment to become superheated. The wood is heated under vacuum to create a pressure gradient, the pressure decreasing toward the surface, to draw the moisture toward the outer surfaces. The moisture quickly converts to water vapor at the reduced pressure and can be condensed or drawn out of the kiln by a vacuum pump as steam during the discharge and moisture removal phase. The humidity and temperature are controlled to allow a certain amount of moisture to remain on the surface of the wood to avoid overdrying and to insure a uniform moisture profile to relieve internal and external stress in the wood throughout the process. While such RF/V systems speed up the wood drying process, they have a high operating cost due to the energy requirements of generating the RF and vacuum pumps. In addition, like the other kiln systems, RF/V is a batch process which is limited in the capacity of wood that can be processed at one time. Accordingly, a need still exists for a system and method of removing moisture from fibrous materials such as sawn and dimensional wood. It is especially desirable for the system and method to operate at a reduced energy and manufacturing cost and in a continuous mode rather than in a batch process
It is even further desirable for a more effective system and method that reduces prolonged drying times associated with conventional kiln and RF Vacuum batch kiln processes. It is even further desirable for the system and method to offer additional commercial and environmental benefits including the prospect of new products that extend our existing timber resources and reduce any unnecessary damage to timber resources that require any such wood drying treatment. It is even further desirable for the system and method to permit and accelerate processing of sawn, dimensional wood or timber such as preservative treatments for generating environmentally friendly end-products.